How are mammalian methionine adenosyltransferases regulated in the liver? A focus on redox stress

Keep a watchful eye on methionine adenosyltransferases, novel therapeutic opportunities for hepatobiliary and pancreatic tumours

Methionine adenosyltransferases (MATs) synthesize S-adenosylmethionine (SAM) from methionine, which provides methyl groups for DNA, RNA, protein, and lipid methylation. MATs play a critical role in cellular processes, including growth, proliferation, and differentiation, and have been implicated in tumour development and progression. The expression of MATs is altered in hepatobiliary and pancreatic (HBP) cancers, which serves as a rare biomarker for early diagnosis and prognosis prediction of HBP cancers. Independent of SAM depletion in cells, MATs are often dysregulated at the transcriptional, post-transcriptional, and post-translational levels. Dysregulation of MATs is involved in carcinogenesis, chemotherapy resistance, T cell exhaustion, activation of tumour-associated macrophages, cancer stemness, and activation of tumourigenic pathways. Targeting MATs both directly and indirectly is a potential therapeutic strategy. This review summarizes the dysregulations of MATs, their proposed mechanism, diagnostic and prognostic roles, and potential therapeutic effects in context of HBP cancers.

Mitochondrial transport and metabolism of the major methyl donor and versatile cofactor S-adenosylmethionine, and related diseases: A review†

S-adenosyl-L-methionine (SAM) is a coenzyme and the most commonly used methyl-group donor for the modification of metabolites, DNA, RNA and proteins. SAM biosynthesis and SAM regeneration from the methylation reaction product S-adenosyl-L-homocysteine (SAH) take place in the cytoplasm. Therefore, the intramitochondrial SAM-dependent methyltransferases require the import of SAM and export of SAH for recycling. Orthologous mitochondrial transporters belonging to the mitochondrial carrier family have been identified to catalyze this antiport transport step: Sam5p in yeast, SLC25A26 (SAMC) in humans, and SAMC1-2 in plants. In mitochondria SAM is used by a vast number of enzymes implicated in the following processes: the regulation of replication, transcription, translation, and enzymatic activities; the maturation and assembly of mitochondrial tRNAs, ribosomes and protein complexes; and the biosynthesis of cofactors, such as ubiquinone, lipoate, and molybdopterin. Mutations in SLC25A26 and mitochondrial SAM-dependent enzymes have been found to cause human diseases, which emphasizes the physiological importance of these proteins.

Metabolism and Regulatory Functions of O-Acetylserine, S-Adenosylmethionine, Homocysteine, and Serine in Plant Development and Environmental Responses

The metabolism of an organism is closely related to both its internal and external environments. Metabolites can act as signal molecules that regulate the functions of genes and proteins, reflecting the status of these environments. This review discusses the metabolism and regulatory functions of O-acetylserine (OAS), S-adenosylmethionine (AdoMet), homocysteine (Hcy), and serine (Ser), which are key metabolites related to sulfur (S)-containing amino acids in plant metabolic networks, in comparison to microbial and animal metabolism. Plants are photosynthetic auxotrophs that have evolved a specific metabolic network different from those in other living organisms. Although amino acids are the building blocks of proteins and common metabolites in all living organisms, their metabolism and regulation in plants have specific features that differ from those in animals and bacteria. In plants, cysteine (Cys), an S-containing amino acid, is synthesized from sulfide and OAS derived from Ser. Methionine (Met), another S-containing amino acid, is also closely related to Ser metabolism because of its thiomethyl moiety. Its S atom is derived from Cys and its methyl group from folates, which are involved in one-carbon metabolism with Ser. One-carbon metabolism is also involved in the biosynthesis of AdoMet, which serves as a methyl donor in the methylation reactions of various biomolecules. Ser is synthesized in three pathways: the phosphorylated pathway found in all organisms and the glycolate and the glycerate pathways, which are specific to plants. Ser metabolism is not only important in Ser supply but also involved in many other functions. Among the metabolites in this network, OAS is known to function as a signal molecule to regulate the expression of OAS gene clusters in response to environmental factors. AdoMet regulates amino acid metabolism at enzymatic and translational levels and regulates gene expression as methyl donor in the DNA and histone methylation or after conversion into bioactive molecules such as polyamine and ethylene. Hcy is involved in Met-AdoMet metabolism and can regulate Ser biosynthesis at an enzymatic level. Ser metabolism is involved in development and stress responses. This review aims to summarize the metabolism and regulatory functions of OAS, AdoMet, Hcy, and Ser and compare the available knowledge for plants with that for animals and bacteria and propose a future perspective on plant research.

SYVN1-MTR4-MAT2A Signaling Axis Regulates Methionine Metabolism in Glioma Cells

Methionine is one of the essential amino acids. How tumor cells adapt and adjust their signal transduction networks to avoid apoptosis in a methionine-restricted environment is worthy of further exploration. In this study, we investigated the molecular mechanism of glioma response to methionine restriction, providing a theoretical basis for new treatment strategies for glioma.

Parenteral nutrition and oxidant stress in the newborn: A narrative review

There is strong evidence that oxidant molecules from various sources contaminate solutions of parenteral nutrition following interactions between the mixture of nutrients and some of the environmental conditions encountered in clinical practice. The continuous infusion of these organic and nonorganic peroxides provided us with a unique opportunity to study in cells, in vascular and animal models, the mechanisms involved in the deleterious reactions of oxidation in premature infants. Potential clinical impacts of peroxides infused with TPN include: a redox imbalance, vasoactive responses, thrombosis of intravenous catheters, TPN-related hepatobiliary complications, bronchopulmonary dysplasia and mortality. This is a narrative review of published data.

Metabolic Dysregulations and Epigenetics: A Bidirectional Interplay that Drives Tumor Progression

Cancer has been considered, for a long time, a genetic disease where mutations in key regulatory genes drive tumor initiation, growth, metastasis, and drug resistance. Instead, the advent of high-throughput technologies has revolutionized cancer research, allowing to investigate molecular alterations at multiple levels, including genome, epigenome, transcriptome, proteome, and metabolome and showing the multifaceted aspects of this disease. The multi-omics approaches revealed an intricate molecular landscape where different cellular functions are interconnected and cooperatively contribute to shaping the malignant phenotype. Recent evidence has brought to light how metabolism and epigenetics are highly intertwined, and their aberrant crosstalk can contribute to tumorigenesis. The oncogene-driven metabolic plasticity of tumor cells supports the energetic and anabolic demands of proliferative tumor programs and secondary can alter the epigenetic landscape via modulating the production and/or the activity of epigenetic metabolites. Conversely, epigenetic mechanisms can regulate the expression of metabolic genes, thereby altering the metabolome, eliciting adaptive responses to rapidly changing environmental conditions, and sustaining malignant cell survival and progression in hostile niches. Thus, cancer cells take advantage of the epigenetics-metabolism crosstalk to acquire aggressive traits, promote cell proliferation, metastasis, and pluripotency, and shape tumor microenvironment. Understanding this bidirectional relationship is crucial to identify potential novel molecular targets for the implementation of robust anti-cancer therapeutic strategies.

Identification of hepatic protein-protein interaction targets for betaine homocysteine S-methyltransferase

Protein-protein interactions are an important mechanism for the regulation of enzyme function allowing metabolite channeling, crosstalk between pathways or the introduction of post-translational modifications. Therefore, a number of high-throughput studies have been carried out to shed light on the protein networks established under different pathophysiological settings. Surprisingly, this type of information is quite limited for enzymes of intermediary metabolism such as betaine homocysteine S-methyltransferase, despite its high hepatic abundancy and its role in homocysteine metabolism. Here, we have taken advantage of two approaches, affinity purification combined with mass spectrometry and yeast two-hybrid, to further uncover the array of interactions of betaine homocysteine S-methyltransferase in normal liver of Rattus norvegicus. A total of 131 non-redundant putative interaction targets were identified, out of which 20 were selected for further validation by coimmunoprecipitation. Interaction targets validated by two different methods include: S-methylmethionine homocysteine methyltransferase or betaine homocysteine methyltransferase 2, methionine adenosyltransferases α1 and α2, cAMP-dependent protein kinase catalytic subunit alpha, 4-hydroxyphenylpyruvic acid dioxygenase and aldolase b. Network analysis identified 122 nodes and 165 edges, as well as a limited number of KEGG pathways that comprise: the biosynthesis of amino acids, cysteine and methionine metabolism, the spliceosome and metabolic pathways. These results further expand the connections within the hepatic methionine cycle and suggest putative cross-talks with additional metabolic pathways that deserve additional research.

Role of glutathione in the regulation of epigenetic mechanisms in disease

Epigenetics is a rapidly growing field that studies gene expression modifications not involving changes in the DNA sequence. Histone H3, one of the basic proteins in the nucleosomes that make up chromatin, is S-glutathionylated in mammalian cells and tissues, making Gamma-L-glutamyl-L-cysteinylglycine, glutathione (GSH), a physiological antioxidant and second messenger in cells, a new post-translational modifier of the histone code that alters the structure of the nucleosome. However, the role of GSH in the epigenetic mechanisms likely goes beyond a mere structural function. Evidence supports the hypothesis that there is a link between GSH metabolism and the control of epigenetic mechanisms at different levels (i.e., substrate availability, enzymatic activity for DNA methylation, changes in the expression of microRNAs, and participation in the histone code). However, little is known about the molecular pathways by which GSH can control epigenetic events. Studying mutations in enzymes involved in GSH metabolism and the alterations of the levels of cofactors affecting epigenetic mechanisms appears challenging. However, the number of diseases induced by aberrant epigenetic regulation is growing, so elucidating the intricate network between GSH metabolism, oxidative stress and epigenetics could shed light on how their deregulation contributes to the development of neurodegeneration, cancer, metabolic pathologies and many other types of diseases.

Ligustrazine modulates renal cysteine biosynthesis in rats exposed to cadmium

The objective of this study was to determine the effect of ligustrazine (TMP) on cadmium (Cd)-induced nephrotoxicity and its relevant mechanism. TMP (50mg/kg) was injected intraperitoneally (i.p.) into rats 1h prior to CdCl₂ exposure (at a Cd dose of 0.6mg/kg). TMP reversed Cd-induced nephrotoxicity, evidenced by the relatively normal architecture of the renal cortex. Additionally, TMP alleviated renal oxidative stress of rats that were exposed to Cd, evidenced by the decreased levels of malondialdehyde (MDA), 4-hydroxynonenal (4-HNE), elevated levels of glutathione (GSH) and GSH/GSSG (glutathione disulfide) ratios. Furthermore, TMP also raised the decreased levels of S-adenosylmethionine (SAM) and cystathionine involved in cysteine biosynthesis in rats exposed to Cd. Further analysis revealed that TMP treatment upregulated expression of several proteins involved in cysteine biosynthesis including methionine adenosyltransferases (MATs) and cystathionine-beta-synthase (CBS). Taken together, these results suggest that TMP remodeled metabolomics of cysteine biosynthesis in rat kidneys and attenuated Cd-induced nephrotoxicity.

Methionine adenosyltransferases in liver health and diseases

Methionine adenosyltransferases (MATs) are essential for cell survival because they catalyze the biosynthesis of the biological methyl donor S-adenosylmethionine (SAMe) from methionine and adenosine triphosphate (ATP). Mammalian cells express two genes, MAT1A and MAT2A, which encode two MAT catalytic subunits, α1 and α2, respectively. The α1 subunit organizes into dimers (MATIII) or tetramers (MATI). The α2 subunit is found in the MATII isoform. A third gene MAT2B, encodes a regulatory subunit β, that regulates the activity of MATII by lowering the inhibition constant (K_i) for SAMe and the Michaelis constant (K_m) for methionine. MAT1A expressed mainly in hepatocytes maintains the differentiated state of these cells whereas MAT2A and MAT2B are expressed in non-parenchymal cells of the liver (hepatic stellate cells [HSCs] and Kupffer cells) and extrahepatic tissues. A switch from the liver-specific MAT1A to MAT2A has been observed during conditions of active liver growth and de-differentiation. Liver injury, fibrosis, and cancer are associated with MAT1A silencing and MAT2A/MAT2B induction. Even though both MAT1A and MAT2A are involved in SAMe biosynthesis, they exhibit distinct molecular interactions in liver cells. This review provides an update on MAT genes and their roles in liver pathologies.

The Oncogene PDRG1 Is an Interaction Target of Methionine Adenosyltransferases

Methionine adenosyltransferases MAT I and MAT III (encoded by Mat1a) catalyze S-adenosylmethionine synthesis in normal liver. Major hepatic diseases concur with reduced levels of this essential methyl donor, which are primarily due to an expression switch from Mat1a towards Mat2a. Additional changes in the association state and even in subcellular localization of these isoenzymes are also detected. All these alterations result in a reduced content of the moderate (MAT I) and high Vmax (MAT III) isoenzymes, whereas the low Vmax (MAT II) isoenzyme increases and nuclear accumulation of MAT I is observed. These changes derive in a reduced availability of cytoplasmic S-adenosylmethionine, together with an effort to meet its needs in the nucleus of damaged cells, rendering enhanced levels of certain epigenetic modifications. In this context, the putative role of protein-protein interactions in the control of S-adenosylmethionine synthesis has been scarcely studied. Using yeast two hybrid and a rat liver library we identified PDRG1 as an interaction target for MATα1 (catalytic subunit of MAT I and MAT III), further confirmation being obtained by immunoprecipitation and pull-down assays. Nuclear MATα interacts physically and functionally with the PDRG1 oncogene, resulting in reduced DNA methylation levels. Increased Pdrg1 expression is detected in acute liver injury and hepatoma cells, together with decreased Mat1a expression and nuclear accumulation of MATα1. Silencing of Pdrg1 expression in hepatoma cells alters their steady-state expression profile on microarrays, downregulating genes associated with tumor progression according to GO pathway analysis. Altogether, the results unveil the role of PDRG1 in the control of the nuclear methylation status through methionine adenosyltransferase binding and its putative collaboration in the progression of hepatic diseases.

Quantitative analyses of the hepatic proteome of methylmercury-exposed Atlantic cod (Gadus morhua) suggest oxidative stress-mediated effects on cellular energy metabolism

BACKGROUND

Methylmecury (MeHg) is a widely distributed environmental pollutant with considerable risk to both human health and wildlife. To gain better insight into the underlying mechanisms of MeHg-mediated toxicity, we have used label-free quantitative mass spectrometry to analyze the liver proteome of Atlantic cod (Gadus morhua) exposed in vivo to MeHg (0, 0.5, 2 mg/kg body weight) for 2 weeks.

RESULTS

Out of a toltal of 1143 proteins quantified, 125 proteins were differentially regulated between MeHg-treated samples and controls. Using various bioinformatics tools, we performed gene ontology, pathway and network enrichment analysis, which indicated that proteins and pathways mainly related to energy metabolism, antioxidant defense, cytoskeleton remodeling, and protein synthesis were regulated in the hepatic proteome after MeHg exposure. Comparison with previous gene expression data strengthened these results, and further supported that MeHg predominantly affects many energy metabolism pathways, presumably through its strong induction of oxidative stress. Some enzymes known to have functionally important oxidation-sensitive cysteine residues in other animals are among the differentially regulated proteins, suggesting their modulations by MeHg-induced oxidative stress. Integrated analysis of the proteomics dataset combined with previous gene expression dataset showed a more pronounced effect of MeHg on amino acid, glucose and fatty acid metabolic pathways, and suggested possible interactions of the cellular energy metabolism and antioxidant defense pathways.

CONCLUSIONS

MeHg disrupts mainly redox homeostasis and energy generating metabolic pathways in cod liver. The energy pathways appear to be modulated through MeHg-induced oxidative stress, possibly mediated by oxidation sensitive enzymes.

Characterization and redox regulation of Plasmodium falciparum methionine adenosyltransferase

As a methyl group donor for biochemical reactions, S-adenosylmethionine plays a central metabolic role in most organisms. Depletion of S-adenosylmethionine has downstream effects on polyamine metabolism and methylation reactions, and is an effective way to combat pathogenic microorganisms such as malaria parasites. Inhibition of both the methylation cycle and polyamine synthesis strongly affects Plasmodium falciparum growth. Despite its central position in the methylation cycle, not much is currently known about P. falciparum methionine adenosyltransferase (PfalMAT). Notably, however, PfalMAT has been discussed as a target of different redox regulatory modifications. Modulating the redox state of critical cysteine residues is a way to regulate enzyme activity in different pathways in response to changes in the cellular redox state. In the present study, we optimized an assay for detailed characterization of enzymatic activity and redox regulation of PfalMAT. While the presence of reduced thioredoxin increases the activity of the enzyme, it was found to be inhibited upon S-glutathionylation and S-nitrosylation. A homology model and site-directed mutagenesis studies revealed a contribution of the residues Cys52, Cys113 and Cys187 to redox regulation of PfalMAT by influencing its structure and activity. This phenomenon connects cellular S-adenosylmethionine synthesis to the redox state of PfalMAT and therefore to the cellular redox homeostasis.

Impact of glutathione supplementation of parenteral nutrition on hepatic methionine adenosyltransferase activity

Background

The oxidation of the methionine adenosyltransferase (MAT) by the combined impact of peroxides contaminating parenteral nutrition (PN) and oxidized redox potential of glutathione is suspected to explain its inhibition observed in animals. A modification of MAT activity is suspected to be at origin of the PN-associated liver disease as observed in newborns. We hypothesized that the correction of redox potential of glutathione by adding glutathione in PN protects the MAT activity.

Aim

To investigate whether the addition of glutathione to PN can reverse the inhibition of MAT observed in animal on PN.

Methods

Three days old guinea pigs received through a jugular vein catheter 2 series of solutions. First with methionine supplement, (1) Sham (no infusion); (2) PN: amino acids, dextrose, lipids and vitamins; (3) PN-GSSG: PN+10 μM GSSG. Second without methionine, (4) D: dextrose; (5) D+180 μM ascorbylperoxide; (6) D+350 μM H₂O₂. Four days later, liver was sampled for determination of redox potential of glutathione and MAT activity in the presence or absence of 1 mM DTT. Data were compared by ANOVA, p<0.05.

Results

MAT activity was 45±4% lower in animal infused with PN and 23±7% with peroxides generated in PN. The inhibition by peroxides was associated with oxidized redox potential and was reversible by DTT. Correction of redox potential (PN+GSSG) or DTT was without effect on the inhibition of MAT by PN. The slope of the linear relation between MAT activity and redox potential was two fold lower in animal infused with PN than in others groups.

Conclusion

The present study suggests that prevention of peroxide generation in PN and/or correction of the redox potential by adding glutathione in PN are not sufficient, at least in newborn guinea pigs, to restore normal MAT activity.

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Methionine adenosyltransferase (MAT) is essential for healthy liver.

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Parenteral nutrition (PN) inhibits hepatic MAT.

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The inhibition is caused by intrinsic peroxides and by unknown component of PN.

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Adding glutathione in PN is not sufficient to prevent PN-associated liver diseases.

Folic acid deficiency induces premature hearing loss through mechanisms involving cochlear oxidative stress and impairment of homocysteine metabolism

Nutritional imbalance is emerging as a causative factor of hearing loss. Epidemiologic studies have linked hearing loss to elevated plasma total homocysteine (tHcy) and folate deficiency, and have shown that folate supplementation lowers tHcy levels potentially ameliorating age-related hearing loss. The purpose of this study was to address the impact of folate deficiency on hearing loss and to examine the underlying mechanisms. For this purpose, 2-mo-old C57BL/6J mice (Animalia Chordata Mus musculus) were randomly divided into 2 groups (n = 65 each) that were fed folate-deficient (FD) or standard diets for 8 wk. HPLC analysis demonstrated a 7-fold decline in serum folate and a 3-fold increase in tHcy levels. FD mice exhibited severe hearing loss measured by auditory brainstem recordings and TUNEL-positive-apoptotic cochlear cells. RT-quantitative PCR and Western blotting showed reduced levels of enzymes catalyzing homocysteine (Hcy) production and recycling, together with a 30% increase in protein homocysteinylation. Redox stress was demonstrated by decreased expression of catalase, glutathione peroxidase 4, and glutathione synthetase genes, increased levels of manganese superoxide dismutase, and NADPH oxidase-complex adaptor cytochrome b-245, α-polypeptide (p22phox) proteins, and elevated concentrations of glutathione species. Altogether, our findings demonstrate, for the first time, that the relationship between hyperhomocysteinemia induced by folate deficiency and premature hearing loss involves impairment of cochlear Hcy metabolism and associated oxidative stress.